This invention relates to alkylation catalysts and in particular to alkylation catalysts containing magnesium oxide or iron oxide, as well as combinations thereof and their methods of preparation. More particularly the invention relates to a method for preparing catalysts containing magnesium oxide or iron oxide for use in ortho-alkylation reactions of hydroxyaromatic compounds. The invention further relates to improvement in the process for the ortho-alkylation of hydroxyaromatic compounds using the above catalyst.
Ortho-alkylation reactions of hydroxyaromatic compounds typically involve vapor phase reaction of an hydroxyaromatic compound, e.g., phenol, with methanol using an alkylation catalyst. Such ortho-alkylated hydroxy aromatic compounds are well known for application as disinfectant, wood preservatives and even as a primary component in certain high-performance thermoplastic products.
U.S. Pat. Nos. 4,554,267; 4,201,880; 3,968,172 and 3,446,856 disclose the use of magnesium-based compounds such as magnesium oxide as a catalyst in the alkylation of the precursor hydroxyaromatic compound with a primary or secondary alcohol. Before the catalyst can be used in the alkylation reaction the catalyst needs to be subjected to calcination to convert the magnesium compound (e.g., magnesium carbonate or magnesium hydroxide) to magnesium oxide. The calcination is usually carried out at temperatures in the range of 350-500° C. Moreover, as discussed in U.S. Pat. No. 4,851,591 the calcination process may be carried out in suitable calcination atmosphere which can be oxidizing, inert, or reducing.
In such ortho-alkylation, it is very important for the catalyst to have high activity i.e. it must have as long and active a life as possible. Moreover, the catalyst should have very good ortho-selectivity. Many of the ortho-alkylation catalysts used are known to produce a mixture that often contains a high proportion of para-alkylated products and consequently such catalysts have marginal commercial utility.
It is known that selectivity and activity are related to the characteristics of the ortho-alkylation catalyst, and to the manner in which it is prepared. U.S. Pat. No. 4,554,267 (Chambers et al) discloses preparation of a magnesium-based catalyst from a slurry process using selected amounts of a copper salt as a promoter. In the process, the magnesium reagent and an aqueous solution of the copper salt are combined to form a magnesium-containing solid phase, which includes uniform, well-dispersed copper. The solid phase is dried, shaped, and calcined. The catalyst system is then used in the alkylation reaction of phenol and methanol. However, such slurry processes are associated with drawbacks such as in the “liquid”—related steps, which involve pre-blending of a copper compound with a magnesium compound and usually require mixing and holding tanks, recirculation piping, and specialized drying systems. Storage of the dried magnesium oxide/copper product (sometimes referred to as a “matrix”) may also be required prior to blending and shaping steps. These steps and the huge investment in time and expense and are of limited commercial utility. Furthermore, the process suffers from unwanted contaminants into the catalyst which affect the desired activity/selectivity of the catalyst.
In addition to the aforementioned wet process, U.S. Pat. Nos. 6,261,987; 6,294,499 and 6,395,871 (Watson et al) describe the preparation of a magnesium-based catalyst using a dry blending method, preferably in the absence of a promoter. The catalyst is especially useful in the alkylation of phenol to manufacture 2,6-dimethyl phenol.
While it is evident from the above that alkylation of hydroxy aromatic compounds using metal oxide catalysts is in general well known and practiced in the art, a need continues to exist for methods and catalysts that have increased selectivity for ortho-alkylation to produce 2,6-dimethyl phenol (also referred to as 2,6-xylenol) with a reduction in the over-alkylation to produce 2,4,6-trimethyl phenol (mesitol). Over-alkylation to mesitol results in higher phenol and methanol usage thereby increasing the overall cost of production of 2,6-xylenol. Moreover, additional purification steps and expense are needed to remove the mesitol and provide a proper disposal method.
Thus, there exists an ongoing need for improvement in catalyst activity/selectivity for ortho-alkylation of hydroxy aromatic compounds so as to favor production of the desired alkylated compound substantially free of unwanted by-products thereby rendering such ortho-alkylation process more productive and cost-effective. As discussed above, the catalyst systems used in the alkylation reaction play an important role in such reactions and to bring about improvements in one or more of the following aspects: catalyst selectivity, catalyst activity, product yield, cost savings, and overall productivity.